An airplane, train, or bus normally has a multiplicity of seats arranged in rows one behind the other. Such seats must provide safe and comfortable sitting places for the passengers. At the same time the seats must be constructed so that the maximum number of passengers can be fitted into the smallest possible space. Normally the law requires that the seats be spaced apart front-to-back by at least 70 centimeters, that is that the distance between a predetermined part of a seat be at least 70 centimeters in front of or behind the same part of the seat in the front or behind it.
Such a seat normally has a limited capacity to tip back so that a passenger need not sit perfectly upright at all times and can rest or sleep. Obviously this tipping-back greatly complicates the space problem, in particular due to the restrictions applied by various regulations.
It is therefore normal to pivot the back part of the seat on the seat part about a horizontal axis extending underneath the lower edge of the back part and at a location normally well below the sitting surface of the seat. The problem with this type of arrangement is that when the back part is tipped back it reduces the space between it and the front of the seat behind it. Thus when a seat is tipped back it can come to lie against the knees of the passenger in the seat behind it, making this passenger uncomfortable and, in fact, infringing upon the space this passenger expects to be able to use. Furthermore when the passenger behind a seat that has been tipped back chooses to tip back his or her seat, the normal result is that this passenger's knees will be moved somewhat forwardly, thereby greatly increasing the extent to which the space between seats is cramped.
Obviously the solution to this problem is to provide the seats at a greater spacing front-to-back. Even when the spacing is only increased by a relatively small distance of, say, 5 to 6 centimeters the result is that a railroad car, airplane, or bus provided with a dozen or more such rows of seats will have one row less, thereby losing valuable revenue.